X-ray phase contrast imaging is an x-ray method that, unlike conventional x-ray devices, exclusively uses the absorption by an object as an information source. X-ray phase contrast imaging combines the absorption with the shift in phase of the x-rays when passing through the object. The information content is disproportionately higher, since the absorption provides accurate images of the significantly absorbing bones, and the phase contrast also produces sharp images of the structures of the soft tissue. This provides the possibility of being able to identify pathological changes, such as the appearance of tumors, vascular restrictions or pathological changes to the cartilage substantially earlier than before.
The passage of x-rays through matter is described by a complex refraction index. The imaginary part of the refraction index specifies the strength of the absorption. By contrast, the real part of the refraction index specifies the phase shift in the x-ray wave passing through a material. In phase contrast imaging, the phase information of the local phase or of the local gradient of the phase of the wavefront passing through an object are determined. Similar to x-ray tomography, tomographic representations of the phase shift may also be reconstructed on the basis of a plurality of images.
A number of possibilities exist in order to realize x-ray phase contrast imaging. The known solutions involve rendering the phase shift in the x-rays during passage through an object visible as an intensity fluctuation using special arrangements and methods. A method is grating-based phase contrast imaging (e.g., Talbot-Lau interferometry), such as is described many times in literature (e.g., in the European patent application EP 1 879 020 A1). Aspects of the Talbot-Lau interferometer are three x-ray gratings that are arranged between an x-ray tube and an x-ray detector.
In addition to the classical absorption image, interferometers of this type may present two additional measurement parameters in the form of further images: the phase contrast image and the darkfield image. The phase of the x-ray wave is determined in this process by interference with a reference wave using the interferometric grating arrangement.
EP 1 879 020 A1 discloses an arrangement according to FIG. 1 having an x-ray tube 1 and a pixelated x-ray detector 2, between which an object 3 to be irradiated is arranged. A source grating G0 (e.g., coherence grating) is arranged between the focal point of the x-ray tube 1 and the object 3. The source grating G0 is used to simulate a number of line sources with spatial partial coherence of the x-rays, thereby forming a precondition for interferometric imaging.
A defraction grating G1, also known as phase grating or Talbot grating, is arranged between the object 3 and the x-ray detector 2. The defraction grating G1 impresses a phase shift by Pi on the phase of the wavefront.
An absorption grating G2 between the defraction grating G1 and the x-ray detector 2 is used to measure the phase shift generated by the object 3. The wavefront upstream of the object 3 is designated W0. The wavefront “distorted” by the object 3 is designated W1. The gratings G0, G1 and G2 must be arranged in parallel and at precise distances from one another.
The x-ray detector 2 is used as locally-dependent proof of x-ray quanta. Since the pixelization of the x-ray detector 2 is generally not sufficient to resolve the interference strips of the Talbot pattern, the intensity pattern is scanned by shifting one of the gratings G0, G1, G2 (“phase-stepping”). The scanning takes place gradually or continuously at right angles to the direction of the x-ray and at right angles to the slot direction of the absorption grating G2. Three different types of x-ray images are recorded and/or reconstructed: the absorption image, the phase contrast image and the darkfield image.
The geometric ratios of the grating arrangement according to EP 1 879 020 A1 are shown schematically in FIG. 2. The gratings G0, G1 and G2 are arranged between the x-ray tube 1 and the planar x-ray detector 2. The source grating G0 has the smallest surface, since it is positioned close to the x-ray tube 1. The absorption grating G2 has the largest surface, since it is positioned directly upstream of the x-ray detector 2.